An umbilical consists of a group of one or more types of elongated active umbilical elements, such as electrical cables, optical fiber cables or tubes, cabled together for flexibility and over-sheathed and/or armoured for mechanical strength and ballast. Umbilicals are used for transmitting power, signals and fluids to and from a subsea installation via the various elongated umbilical elements. An increasingly important use of umbilicals is the transmission of electrical power to electrical devices on the seabed, and depths of up to 2000 meters are common.
Generally the elements are arranged within the umbilical as symmetrically as possible. The cross-section is preferably circular. In order to fill the interstitial voids between the various umbilical elements and obtain the desired circular configuration, filler components may be included within the voids.
Umbilicals have to be able to withstand substantial laying and service loads and load combinations, and perform their functions for an extended design life.
Electrical and/or optical cables are not designed to resist the loads applied to the umbilical. These hoses and tubes, generally made of thermoplastic material are designed merely to resist collapse.
The elements are normally wound in a helical pattern around a central core. The core may be a larger steel tube, or one of the umbilical elements for instance a power cable. With the helically wound elements, such an umbilical under normal conditions will be able to withstand the moderate loads to be applied to it without the addition of substantial armouring layers.
However, under severe conditions such as in use in deep water and/or in dynamic applications increased loads will be applied to the umbilical, due to the weight of the umbilical and to the dynamic movement of water. Strengthening elements and ballast elements have to be added to the umbilical to withstand these loads.
Armoured cables to be supported from an offshore platform are known from GB 2 183 402.
U.S. Pat. No. 6,472,614 discloses an umbilical comprising a plurality of steel tubes helically wound around a core and at least one substantially solid steel rod helically wound around said core, said rod being shaped and sized for absorbing tensile loading on said umbilical. The steel rod is arranged in a void between the steel tubes. The umbilical comprises at least one elongated umbilical element selected from the group consisting of optical fiber cables, electrical power cables and signal cables.